Solid oxide electrolyte fuel cell

Abstract: An improved structure of a solid oxide electrolyte fuel cell (SOFC) is disclosed. A generating layer (21) in the SOFC consists of three layers of a fuel electrode (23), an oxide layer (22) and an oxygen electrode (24), and is shaped into a dimpled structure having recessed portions (25) on its both surfaces. Protruded portions (26a) on the side of the fuel electrode (23) are electrically bonded with an interconnector layer (28) via a first conductive adhesive (27), while protruded portions (26b) on the side of the oxygen electrode (24) of another generating layer (21) are electrically bonded with the same interconnector layer (28) via a second conductive adhesive (29). A plurality of generating layers (21) and a plurality of interconnector layers (28) are alternately stacked and they are electrically bonded in the above-described manner. Preferably the first conductive adhesive (27) is made of material common to the fuel electrode (23) and the second conductive adhesive (29) is made of material common to the oxygen electrode (24). Owing to the improved structure, occurrence of lateral flows of electrons in the interconnector layers (28) having a large resistance can be suppressed, hence an electrical resistance can be reduced, and also amounts of materials of the oxygen electrode and the fuel electrode can be made small.

Description: BACKGROUND OF THE INVENTION: 1. Field of the Invention: The present invention relates to a solid oxide electrolyte fuel cell (SOFC), and more particularly to an SOFC available, besides for power generating purpose, as an electrolyte cell of hydro-electrolysis, CO 2 -electrolysis or the like. 2. Description of the Prior Art: As an SOFC in the prior art, a construction proposed by the applicant of this application and disclosed in Laid-Open Japanese Utility Model Specification No. 4-8259 (1992), has been known. Fig. 3 in the accompanying drawings of the present application is equivalent to Fig. 1 in this laid-open U.M. specification, which shows an outline of the proposed construction. In Fig. 3, reference numeral 1 designates a generating layer composed of a solid oxide layer 2 and a fuel electrode 3 and an oxygen electrode 4 respectively disposed on the opposite sides of the solid oxide layer 2. Above and under the generating layer 1 are disposed laminated layers 8a and 8b each consisting of an interconnector layer 5 and an oxygen electrode 6 and a fuel electrode 7 respectively laminated on the opposite sides of the interconnector layer 5. An outside crest portion 9 of the fuel electrode 3 on the upper side of the generating layer 1 is bonded with the fuel electrode 7 of the upper laminated layer 8a, and a fuel passage 10 is formed of a space reserved therebetween. On the other hand, an outside crest portion 11 of the oxygen electrode 4 on the under side of the generating layer 1 is bonded with the oxygen electrode 6 of the lower laminated layer 8b, and an oxidizer passage 12 is formed of a space reserved therebetween. As explained in the above-referred laid-open Japanese U.M. specification, in contrast to the structure so far considered a main current, the above-described structure is a really important proposal in this technical field in that members not directly concerned in power generation such as support members interposed between the generating layer 1 and the laminated layers 8a and 8b are made unnecessary. It is to be noted that while an SOFC has a power generation efficiency exceeding 60% and is ranked at an important level in view of measures for energy control, a countermeasure such as reduction of a manufacturing cost is necessitated. Although the SOFC structure in the prior art shown in Fig. 3 achieved an important improvement in this respect, it still involved a problem that the oxygen electrodes 6 and the fuel electrodes 7 forming the laminated layers 8a and 8b, which are not directly concerned in power generation per se, were necessary. In the above-described construction, since the generating layer 1 employed a dimpled structure, electrons produced in the generating layer 1 would concentrate to a dimpled portion (recessed portion), and hence, if the dimpled portion is in itself directly bonded with the interconnector layer, an electric resistance becomes large. The reason for the necessity of the oxygen electrodes 6 and the fuel electrodes 7 forming the laminated layers 8a and 8b is because it was intended to bring the dimpled portions into contact with the corresponding electrode 6 or 7 so that the produced electrons can diffuse and a lateral flow of electrons can be generated. Solid electrolyte fuel cells having flat anode/electrolyte/cathode layers (AEC-layers) connected by interconnectors with ribs and fluid passages are described in JP-22 04 974 and JP-31 16 659. The interconnectors are bonded to the interposed AEC-layers by means of conductive adhesives made from either the anode or the cathode material, depending on the side of contact. A power generating device housed in a box-type housing container is described in JP-40 56 075, wherein in a flat type solid electrolytic fuel cell, dimples are provided in any one of a main surface of a power generator and interconnectors so that a gas flow passage is formed in the diagonal direction of the housing container. The idea of providing recesses and projections in the solid electrolyte plate to increase a surface area and arranging flat current collectors on both sides of the solid electrolyte plate so as to be in contact with the peaks of the projections and recesses for constituting a cell is described in JP-22 76 166. The unit cells formed in this manner are stacked in order to be connected in series. SUMMARY OF THE INVENTION: It is the object of the present invention to provide a solid electrolyte fuel cell (SOFC) including at least two dimpled generating layers each formed in a three layer structure which has its electric resistance minimized and which can be manufactured at reduced cost. According to the present invention there is provided a solid electrolyte fuel cell including at least two generating layers each consisting of three layers of a fuel electrode, an oxide layer and an oxygen electrode and formed in a dimpled structure provided with recessed portions on its both surfaces; wherein protruded portions on the side of the fuel electrode outside of said recessed portions of one generating layer are electrically bonded with an interconnector layer via a first conductive adhesive, and protruded portions on the side of the oxygen electrode outside of said recessed portions of another generating layer disposed on the opposite side of the same interconnector layer are electrically bonded with the same interconnector layer via a second conductive adhesive such that the protruded portions are opposed to each other. Generally, in a SOFC, since yttria-stabilized-zirconia (hereinafter abbreviated as YSZ) formed as ceramics is employed for the solid oxide layer, contrivance is made so as to prevent cracking of the solid oxide layer caused by thermal deformation of the generating layer composed of cell component members having different coefficients of thermal expansion and constrained by one another by equalizing the coefficients of thermal expansion of the respective cell component members. Therefore, in the SOFC according to the present invention also, it is required to use fuel electrode material, oxygen electrode material and interconnector material having coefficients of thermal expansion matched to that of the solid oxide layer, and materials having negligibly small specific electric resistances are employed. As representative examples of the materials, the following materials are available on the basis of results of experiments conducted by the inventors of the present invention: interconnector material: LaSrCrO 3 (1 mm in thickness) fuel passage side: conductivity 1 S/cm, resistance 0.1 Ω·cm 2 oxidizer passage side: conductivity 30 S/cm, resistance 0.003 Ω·cm 2 average: conductivity 10 S/cm, resistance 0.01 Ω·cm 2 oxygen electrode material: LaSrMnO 3 (50 µm in thickness) conductivity 20 S/cm, resistance 2.5 x 10 -4 Ω·cm 2 fuel electrode material: Ni/YSZ (60:40) (50 µm in thickness) conductivity 500 - 1000 S/cm, resistance 1 - 0.5 x 10 -5 Ω·cm 2 The numerals of resistance above all indicate resistance values per unit area as measured in the thicknesswise direction. As will be apparent from the above data, partly from the reason that the interconnector layer is necessitated to have a thickness enough for in itself reserving a mechanical strength as a structural member, the interconnector layer has a predominantly high electric resistance, and so, it is necessary to minimize the length of this electric flow passage. Furthermore, if the above-described data are applied to an electric flow passage schematically shown in Fig. 4, when a pitch of dimples is chosen to be 3 mm, that is, a lateral path length of electrons is 1.5 mm, a thickness of an interconnector layer 5 made of LaSrCrO 3 is 2 mm and thicknesses of a fuel electrode made of Ni/YSZ and an oxygen electrode made of LaSrCrO 3 are varied, an overall resistance of the three-layer structure would change as indicated in the following Table-1: Thickness of Oxygen Electrode 50 µm 100 µm 150 µm 200 µm Thickness of Fuel Electrode 50 µm 0.25 0.14 0.10 0.085 100 µm 0.25 0.14 0.10 0.081 150 µm 0.25 0.14 0.10 0.080 200 µm 0.25 0.13 0.10 0.079 Unit: Ω·cm 2 As will be obvious from Table-1 above, while the thickness on the fuel electrode side could be chosen to be 50 µm at maximum, on the oxygen electrode side an electric resistance largely depends upon the thickness, and so, in order to reduce an electric resistance the thickness must be chosen thick. Whereas, according to the present invention, since a resistance caused by lateral flows of electrons in an electrode is not present, upon connecting an oxygen electrode of a generating layer with an interconnector layer, it is only necessitated to apply a relatively thin adhesive (normally the same material as the oxygen electrode is used) to such extent that electrical connection can be made. As described above, in the SOFC according to the present invention, an electric resistance can be reduced, and also amounts of materials of an oxygen electrode and a fuel electrode can be made small. Therefore, the present invention can provide an improved solid oxide electrolyte fuel cell having a power generation efficiency enhanced and having a manufacturing cost reduced. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of one preferred embodiment of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1 is a schematic cross-section view of an SOFC according to one preferred embodiment of the present invention; Fig. 2 is a schematic perspective view of a generating layer in the SOFC shown in Fig. 1; Fig. 3 is a schematic cross-section view of one example of an SOFC in the prior art; and Fig. 4 is a diagrammatic cross-section view showing flows of electrons in the SOFC shown in Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT: Now, the present invention will be described in greater detail in connection to one preferred embodiment of the invention illustrated in Figs. 1 and 2. In these figures, reference numeral 21 designates a generating layer, in which a fuel electrode 23 and an oxygen electrode 24 are respectively formed on the opposite surfaces of a solid oxide layer 22. On the both sides of the generating layer 21 are formed a large number of dimples (recessed portions) 25, and on the opposite side to the recessed portions of the dimples 25 are formed protruded portions 26a and 26b. A protruded portion 26a on the fuel electrode side of one generating layer 21 is bonded with an interconnector layer 28 via a first conductive adhesive 27 made of material common to the fuel electrode 23. On the other hand, a protruded portion 26b on the oxygen electrode side of another generating layer 21 is bonded with the same interconnector layer 28 via a second conductive adhesive 29 made of material common to the oxygen electrode 24 at a position opposed to the protruded portion 26a on the fuel electrode side of the first-mentioned generating layer 21. In addition, reference numeral 30 designates a space region surrounded by the generating layer 21, the conductive adhesives 27 and the interconnector layer 28, that is, a fuel passage for feeding fuel gas to the fuel electrode 23. Likewise, reference numeral 31 designates a space region surrounded by the generating layer 21 (secondly referred to), the conductive adhesives 29 and the interconnector layer 28, that is, an oxygen passage for feeding fuel gas to the oxygen electrode 24. In the SOFC according to the illustrated embodiment of the present invention, occurrence of lateral flows of electrons in the interconnector layer 28 having a large resistance can be suppressed, owing to the novel construction that generating layers 21 composed of a solid oxide layer 22 having a fuel electrode 23 and an oxygen electrode 24 formed respectively on its opposite surfaces and interconnector layers 28 are disposed alternately, protruded portions 26a on the fuel electrode side of the generating layer 21 are bonded with the interconnector layer 28 via a first conductive adhesive 27 made of material common to the fuel electrode 23, also protruded portions 26b on the oxygen electrode side of another generating layer 21 disposed on the opposite side of the same interconnector layer 28 are bonded with the same interconnector layer 28 via a second conductive adhesive 29 made of material common to the oxygen electrode 24, and moreover, the bonding portion between the first conductive adhesive 27 and the interconnector layer 28 and the bonding portion between the second conductive adhesive 29 and the same interconnector layer 28 are positioned as opposed to each other.

Claims: A solid electrolyte fuel cell including at least two generating layers (21) each consisting of three layers of a fuel electrode (23), an oxide layer (22) and an oxygen electrode (24) and formed in a dimpled structure provided with recessed portions (25) on its both surfaces; wherein protruded portions (26a) on the side of the fuel electrode (23) outside of said recessed portions (25) of one generating layer (21) are electrically bonded with an interconnector layer (28) via a first conductive adhesive (27), and protruded portions (26b) on the side of the oxygen electrode (24) outside of said recessed portions (25) of another generating layer (21) disposed on the opposite side of the same interconnector layer (28) are electrically bonded with the same interconnector layer (28) via a second conductive adhesive (29) such that the protruded portions (26a,26b) are opposed to each other. A solid oxide electrolyte fuel cell as claimed in claim 1, characterized in that said first conductive adhesive (27) is made of material common to said fuel electrode (23), and said second conductive adhesive (29) is made of a material common to said oxygen electrode (24).

Inventors: MIYAMOTO HITOSHI, NANJO FUSAYUKI, SATAKE TOKUKI, TAKENOBU KOICHI, WATANABE KIYOSHI, MIYAMOTO, HITOSHI, NANJO, FUSAYUKI, SATAKE, TOKUKI, TAKENOBU, KOICHI, WATANABE,KIYOSHI, Miyamoto, Hitoshi, c/o Takasago Res.&amp; Dev.Center, Nanjo, Fusayuki, c/o Kobe Shipyard &amp; Mach.Work of, Satake, Tokuki, c/o Kobe Shipyard &amp; Mach.Work of, Takenobu, Koichi, c/o Kobe Shipyard &amp; Mach.Work of, Watanabe,Kiyoshi, c/o Kobe Shipyard &amp; Mach.Work of
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1994
Main classification: H01M8
IPC codes: H01M8